There are two basic techniques for converting an analog signal into a digital signal: open loop and feedback. An open loop analog-to-digital (A/D) converter generates a digital signal directly in response to an analog input signal. The quality of an open loop A/D converter depends upon several factors, including sampling rate, quantization error, aliasing, and component matching.
A sigma-delta A/D converter (ΣΔ A/D converter) is a feedback type of converter. The ΣΔ A/D converter has a ΣΔ modulator that uses over-sampling (having a high sampling rate; at least above the Nyquist rate) and filtering to develop a high signal-to-noise-plus-distortion ratio (SNDR) in the signal band, and achieves high resolution by precise timing.
A ΣΔ A/D converter typically includes a ΣΔ modulator having a quantizer and a decimator. A ΣΔ modulator processes an analog signal through one or more cascaded integration stages. A weighted feedback (or feed-forward) signal is subtracted from the analog signal between integration stages. The signal difference, also known as an error signal, represents the original analog signal. For example, if the error signal is consecutively positive, then it may be inferred that the input signal is rising rapidly. After the last integrator, the error signal is quantized and filtered to form a digital output signal.
The feedback technique provides a ΣΔ A/D converter with error correction capability under most circumstances. As is the case with most linear circuits, the performance of a ΣΔ A/D converter is affected by circuit noise and dependent on timing variations. For example, external sources may add noise to an input signal, and thereby escalate signal peaking. An escalated input signal may be too large for an integrator in the analog-to-digital converter and the higher amplitudes at the integrator output may be cut-off, the result may be signal distortion and the A/D converter may provide an inaccurate or poor digital output signal. Component variations also contribute to noise and may also lead to decreased performance. In addition, circuit non-linearities cause delay and decrease the performance of the ΣΔ A/D loop. It would be desirable to reduce the effects of noise and component variations in ΣΔ A/D converters. It would also be desirable to compensate for loop delay caused by circuit non-linearities.